The present invention relates generally to liquid crystal display devices with direction-constant optical anisotropy, provided by irradiation of polarized light onto the surface of an organic high-molecular or polymeric film or the like, and to polarized light irradiation methods for irradiating polarized light for provision of such optical anisotropy, as well as apparatus used therefor. More particularly, the invention relates to a polarized light irradiation method and apparatus for providing added liquid crystal orientation controllability by irradiation of polarized light onto an orientation film constituting more than one liquid crystal display element.
Liquid crystal display devices have been widely used as devices for displaying a variety of images, including still images and motion pictures.
The liquid crystal display devices of this type are basically configured from a pair of substrates with a liquid crystal layer disposed between them to thereby constitute what is called a "liquid crystal panel" structure, wherein at least one of the substrates is made of an optically transparent material, such as glass or the like. The liquid crystal display devices may be categorized into two forms, one of which is arranged to turn-on and turn-off of a specified pixel by selectively applying a voltage to a variety of types of electrodes for pixel formation as formed on the substrate of the liquid crystal panel, and the other of which is to effect the turn-on/off of a certain pixel by forming the various types of electrodes and an active element for pixel selection and then selecting this active element.
Especially, the liquid crystal display devices of the latter type are called "active matrix" type devices, and are the major industry-leading devices of the currently available liquid crystal display devices, due to the high contrast and high-speed visualizability and other advantages they offer. Most prior known active-matrix type liquid crystal display devices employ the so-called "longitudinal electric field" scheme in which an electric field is applied between electrodes formed on one substrate and those formed on the other substrate, which electric field is used for changing the orientation direction of the liquid crystal layer disposed between the substrates.
In recent years, however, a liquid crystal display device of the type employing a so-called "lateral electric field" scheme (also known as the IPS system) has been developed, in which the direction of an electric field being applied to the liquid crystal layer is substantially parallel to the substrate surface. There is a known liquid crystal display device of the lateral electric field type which is arranged to make use of a comb-shaped electrode configuration for one of the two substrates, thereby obtaining a drastically widened viewing angle (Japanese Patent Disclosure No. 63-21907, and U.S. Pat. No. 4,345,249).
On the other hand, as one representative example of methods for forcing the liquid crystal molecules constituting a liquid crystal layer to face or point toward a specified direction, a technique has been reduced to practice for forming on a substrate an organic orientation film made of an organic high polymer film, such as, for example, a polyimide-based film or equivalents thereto, which film is then subjected to rubbing treatment, thereby attributing liquid crystal orientation control functions thereto.
A method (optical orientation) is also known for achieving such liquid crystal orientation controllability by irradiation of light upon the orientation film of a polyimide-based organic high-molecular film or the like as formed on a substrate (see U.S. Pat. No. 4,974,941; Published Japanese Patent Application or "PJPA" No. 5-34699; PJPA No. 6-281937; PJPA No. 7-247319).
However, these prior art optical orientation techniques have not been applied to the lateral electric field type system, and no consideration has been given to the achievability of specific effects and advantages obtainable from application of such optical orientation techniques to the lateral electric field type system, which is different in design concept from the longitudinal electric field type system, in that the various types of electrodes for pixel formation are formed on only one substrate in the lateral electric field type system.
Prior art orientation methods using rubbing techniques for orienting liquid crystal molecules of the liquid crystal panel in a specified direction involve processing methods in which cloth is brought into direct contact with the orientation films, which can possibly produce the risk of unwanted generation of static electricity and cause contamination of such orientation films during such rubbing processing.
Generation of static electricity on the orientation film can result in destruction of the switching elements, such as TFTs and the like, and also can lead to a change in switching characteristic. Contamination of the orientation film would result in a local irregularity of the frequency dependency of a threshold voltage and a decrease in voltage hold rate, or alternatively, a change in pre-tilt angle and a change in liquid crystal orientation.
Further, as the substrate size increases, it becomes more difficult to adequately control the force applied during rubbing over the entire substrate area, which in turn leads to the risk that scars or other surface irregularities can occur due to rubbing on a large-size substrate.
Another problem faced by the prior art is that fine cutting particles or chips can be produced from a film being rubbed with cloth, which might serve as a significant dust source inside a cleanroom during the manufacture of liquid crystal display devices, thereby greatly reducing the production yield at the other manufacturing process steps.
A further problem lies in the fact that, since the substrate inherently has a somewhat irregular surface configuration due to the presence of electrodes and active elements (switching elements such as TFTs and the like) formed thereon, such surface configuration results in the presence of several incompletely rubbed portions on the resulting substrate surface after rubbing treatment has been performed, which portions undesirably result in a lack of light--known as optical dislocation--in a back display, which leads to a decrease in contrast.
The aforementioned problems become more severe as the substrate increases in size.
On the other hand, unlike the above-mentioned orientation processing methods using such contact schemes, an optical orientation technique is known which is capable of providing the intended liquid crystal orientation controllability without effecting control with the surface of an organic high-molecular film being used. This method involves the irradiation of polarized light having a certain polarizing axis onto an organic high polymer film thereby adding thereto the orientation control functionality in a way corresponding to the axis of polarized light thereof.
However, this approach is still faced with a potential problem as to how the light is radiated onto a practically implemented substrate of liquid crystal display elements in order to attain the same required high efficiency as that of the rubbing method or the like, while at the same time retaining the ability to uniformly irradiate rays of light thereon. Especially, as presently available liquid crystal display devices increase in size and dimension, the region in which the liquid crystal needs to be oriented tends to increase more and more. The technical trend in currently available liquid crystal display devices is to achieve an effective area which is ten (10) inches in diagonal measure or wider; and, in near future, a size exceeding 10 inches will become the major technological approach. Unfortunately, the efficiency presently achieved remains considerably less than that of the rubbing treatment because of the fact that the region is too wide to irradiate polarized light for optical orientation. In regard to a polarized light irradiation apparatus, no such apparatus exists which is capable of simultaneous exposure of a large-size substrate measuring 10 inches or greater. Currently, the size limit of optical irradiation is 7 to 8 inches in diagonal dimension. Accordingly, it remains impossible to apply the optical orientation technique with an efficiency equivalent to that of the rubbing treatment to those substrates which are 10 inches or greater in size; although, such technique is expected to be the major approach in the future in the manufacture of liquid crystal display devices.